Known pressure sensors of the generic type contain only a capacitive measuring element consisting of a support plate and a membrane, which carry electrically conductive layers. Although measurement can be carried out independently of the type of gas and with high accuracy between about 10.sup.-6 mbar and 10 bar by means of such measuring elements, the total range cannot be measured with a single measuring element.
The use of pressure sensors which are in the form of heat conduction manometers, for example according to Pirani, is also known. In such pressure sensors, at least one measuring wire is electrically heated and the pressure is determined from the heating power, making use of the pressure-dependent thermal conductivity of the gas. In this way, it is possible to measure the pressure in a range between about 10.sup.-3 mbar and a few 100 mbar. Above a few 10 mbar, however, convective heat transmission predominates, so that the measurement is influenced there by gas flows and is highly position-dependent. Moreover, the measurement by this method is always dependent on the type of gas. A Pirani pressure sensor, including its evaluation electronics, can also be designed in such a way that it can be operated up to about 10.sup.-5 or max. 10.sup.-6 mbar, but in this case higher pressures above a few mbar can no longer be reliably measured.
It is also known that so-called ionization vacuum sensors whose function is based on the measurement of the particle current density after striking of a gas discharge can be used for pressures below 10.sup.-2 mbar and, with further reduced accuracy, up to 10.sup.-1 mbar. A distinction is made between cold-cathode ionization vacuum gages and those having a hot cathode. They are not capable of functioning in higher pressure ranges and are inaccurate from about 10.sup.-2 mbar. They are in principle dependent on the type of gas.
If it is intended to measure large pressure ranges, for example from about 10.sup.-6 mbar to about 100 mbar, it is usual to use at least two different, spatially separated pressure sensors which, independently of one another, are each also provided with devices for processing the measured signal. Thus, for example, it is possible to use two or more pressure sensors which each contain a capacitive measuring element of the type described at the outset which is suitable for measuring a part of the range. These and similar solutions are, however, more expensive owing to the associated technical complexities.
However, the combination of different pressure sensors in one apparatus is also known. Thus, EP-A-0 658 755 discloses a pressure measuring apparatus in which a Pirani element and a cold-cathode element are combined to give a pressure measuring apparatus, the former measuring an upper pressure range and the latter a lower one. Although the apparatus is compact and can measure the total above-mentioned range, it has the above-mentioned disadvantages of the stated elements in terms of measuring technology.
The combination of a bellows-type mechanical pressure measuring element with a Pirani-like pressure measuring element is also known (U.S. Pat. No. 3,064,478). Here, the corresponding pressure measuring apparatus is relatively inconvenient. Moreover, uncertainities in the measured result may arise from the spatial separation of the measuring elements.